The present invention relates generally to the coating of a work piece with an electrophoretic coating material. More particularly, the present invention concerns method and apparatus for electrophoretically coating a conductive work piece, such method comprising the steps of: establishing the work piece at one electrical potential, flowing an electrophoretic coating in a linear stream corresponding to the linear dimension and in close proximity to the work piece surface, imparting an electrical charge to the electrophoretic coating, impinging the charged linear stream of electrophoretic coating onto the work piece surface and moving the work piece and charged linear stream relative to one another and in a direction lateral to the selected linear dimension, to electrophoretically coat the entirety of such work piece surface. The present invention has particular application in the coating of internal and external surfaces of containers, such as cans, and other objects, such as heat exchangers, radiators, drums, automobile wheels, automobile oil filter caps and the like.
Electrophoresis generally concerns the movement of ionic particles within an aqueous system in response to electrical charges imparted to such system. Negatively charged particles or ions in such an aqueous solution (i.e., an anodic coating) migrate in response to such electrical potential to any positively charged conductor which may be immersed in the solution for deposit thereon. Positively charged particles or ions (i.e., cathodic coating materials) likewise migrate and are deposited upon a negatively charged conductor within the coating bath.
Typically, an electrical potential in the range of approximately 100 to 500 volts has been used for electrophoretic coating. The thickness, and hence durability, of such electrophoretically deposited coating layer is dependent upon a number of factors, including, inter alia, the voltage used, the separation between the anode and cathode, the length of time coating is permitted to continue, the pH of the coating solution, the characteristics of the coating polymer used and the conductivity of the particular material then being coated. During coating, after some coating particles have been deposited upon the conductive surface being coated, there is a gradual reduction in the conductivity thereof and the work piece being coated becomes increasingly insulated. When the thickness of the electrophoretically deposited coating layer becomes sufficiently thick for a given system, the previously conductive surface becomes insulated to the extent that no further substantial electrodeposition will occur. Similarly, if some portion of the surface of the work piece to be coated has been previously coated with an insulating coating, further electrodeposition on that coated surface will occur only with higher voltages, closer proximity of electrodes, more conductive coating materials, longer coating times or other changes to the system.
Various of the above techniques of electrophoretic coating have been used heretofore in the art. Those techniques have had a number of disadvantages associated therewith. In many such prior art electrophoretic coating techniques, it has been necessary to immerse the work piece into a coating bath, which has necessitated large capital outlays for the often spacious tanks required to accommodate the work piece therein. Also, such immersion techniques have been found to require an excessive amount of time and extra mechanical equipment to accomplish such dipping or immersion.
A further serious disadvantage of such prior art immersion techniques is the necessary result that both the internal and external surfaces of the work piece to be coated must be done simultaneously. This is especially undesirable when, as is often the case, either the exterior surface or the interior surface thereof should not or does not need to be coated, or when different types of coating are required for the interior and exterior surfaces. The waste involved and lack of product flexibility constitute in many cases debilitating disadvantages so severe that other, even more expensive, techniques may become necessary.
Another technique used heretofore has been the electrodeposition of coating materials on sheet metal prior to its being fabricated into a particular coated body. Such techniques have resulted in exposed and/or uncoated breaks in the coating, which have occurred during the various fabrication steps, such as stamping, welding, heating, etc. Such unprotected areas may be especially undesirable in the container industry, or in other industries where bare metal will constitute a safety hazard or economic loss.
Yet another prior art technique, that of electrostatic spraying, has been used for various commercial coating operations. A number of further disadvantages have also resulted from the use of those techniques. Such spray techniques have required difficult adjustments and excessive maintenance problems. Further, in electrostatic spraying techniques a relatively thick coating has been required to insure complete coverage of the surface to be coated. Yet further, spraying techniques have been expecially difficult to utilize where the coating of an irregular and/or interior surface has been required.
Inversion flooding has been suggested as a technique for electrocoating of the interior surface of wide mouthed containers. That process is suitable for columnar containers and contemplates inverting the can and inserting upwardly and into the can opening a mating prod with a diameter which closely matches the internal diameter of the can. Electrodeposition coating material is then force-pumped into the can from the top of the prod so as to flood the constricted space between the prod and the can from the top, down along the sides, and out the bottom, thereby to coat the surface. Such a system is limited inherently to coating the interior of containers and, because of the force flooding in the constricted space unless the constrictions are uniform and continous the coating thickness will vary and may be striped.
Accordingly, in view of the shortcomings of the prior art, it is an object of the present invention to provide method and apparatus for electrocoating wherein the problems and disadvantages associated with the prior art may be materially reduced or avoided.
It is a further object of the present invention to provide method and apparatus for electrocoating wherein a work piece may be coated selectively on the interior or exterior surfaces thereof.
It is an additional object of the present invention to provide means for relative movement between an electrically charged work piece and a linear stream of oppositely charged coating, whereby either may be fixed and the other moved in alternative embodiments to permit uniform application of coating onto the entirety of the surface to be coated.
It is a further object of the present invention to provide means for relative motion between an electrically charged work piece having a selected linear dimension and an oppositely charged linear stream of electrophoretic coating material where such relative motion is lateral to such selected linear dimension.
It is a yet further object of the present invention to provide relative rotation between a work piece at a fixed potential, having an axis of symmetry, and a proximately disposed electrode from which flows a charged linear stream of coating material, whereby the entirety of such surface may be electrophoretically coated.
These and other advantages and objects of the present invention will become apparent to those skilled in the art in view of the following specification setting forth in greater detail the preferred embodiments of the present invention.